1. Field of the Invention
The present invention relates to a control unit suitable for facilitating movement of an object connected thereto between desired positions and relates particularly, but not exclusively, to a control unit in the form of an hydraulically operated piston operably connectable to a roller mechanism in an infinitely-variable-transmission (IVT) or the like. The invention also relates to an IVT incorporating said control unit.
2. Background Art
Several patent publications of recent years, of which WO90/05860 is an example, describe infinitely-variable-transmissions in which direct connection of each roller to a piston subjected to controlled hydraulic pressure suffices to maintain the angular orientation of each roller—and thus the ratio transmitted by it from an input disc to an output disc—at the appropriate value at all times. The key to the efficacy of such an apparently simple method of roller control is that the torque transmitted by the variator can be controlled by adjustment of the hydraulic pressure. A control system receives inputs representative of all the essential parameters, and sets up within the cylinder the appropriate hydraulic pressure to match the torque reaction associated with the desired orientation, so causing the roller to seek and hold that orientation.
One of the problems encountered in practice by IVTs in which rollers are controlled is that the length of the stroke that the piston must execute in order fully to control the roller is relatively long. Consequently, the housing for the piston/cylinder is of such length that it can be difficult to install within the tight confines of some engine or transmission compartments.
A solution to the above-mentioned problem is presented in WO 99/54646 in which there is described a roller control unit for a continuously-variable-ratio transmission of the toroidal—race roller traction type, in which the unit comprises a piston/cylinder arrangement characterised by telescopic shrouds of variable length extending between the piston and the end wall. The shrouds act to define, in combination with the piston and cylinder, a chamber for receiving hydraulic actuation fluid and translate between deployed and retracted positions as and when the piston moves. The provision of telescopic shrouds facilitates a reduction in the axial length of the actuation mechanism which is of significant benefit when installing in the tight confines of some engine or transmission compartments.